Downhole Mills and Improved Cutting Structures

ABSTRACT

A drill bit for cutting through a downhole metal structure includes a bit body having a central axis and a bit face. The bit body is configured to rotate about the central axis in a cutting direction. In addition, the bit includes a cutting structure disposed on the bit face. The cutting structure includes a plurality of circumferentially spaced blades and a plurality of primary cutter elements mounted to each blade. Each primary cutter element has a forward-facing primary cutting face. Each primary cutter element is made of a whisker ceramic composite.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 35 U.S.C. §371 national stage application of PCT/US2013/061556 filed Sep. 25, 2013 and entitled “Downhole Mills and Improved Cutting Structures,” which claims benefit of U.S. provisional patent application Ser. No. 61/705,271 filed Sep. 25, 2012, and entitled “Downhole Mills and Improved Cutting Structures,” both of which are hereby incorporated herein by reference in their entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND

The invention relates generally to downhole cutting devices. More particularly, the invention relates to mills and bits with improved cutting structures for cutting through downhole metal structures such as casing.

In some cases, previously drilled and cased wellbores become non-productive. When such a wellbore becomes unusable, and there are sufficient hydrocarbon reserves in the surrounding formation to justify continued production, a new borehole may be drilled in the vicinity of the existing cased borehole or alternatively, a new borehole may be sidetracked near the bottom of a serviceable portion of the cased borehole. Sidetracking from an existing cased borehole can also be used to access multiple production zones from a common wellbore.

Sidetracking is often preferred because it reduces drilling, casing and cementing needs, as well as associated costs. Sidetracking is typically accomplished by either milling out an entire section of casing followed by drilling a lateral borehole into the exposed borehole sidewall, or by milling through the side of the casing with a mill guided by a wedge or “whipstock” component followed by drilling a lateral borehole through the hole in the casing.

Drilling a side tracked hole through casing made of steel is challenging and often results in unsuccessful penetration of the casing. In addition, if the window is improperly cut, a severely deviated dog leg may result rendering the sidetracking operation unusable.

One conventional approach to drilling through steel casing for sidetracking is to employ a bit or mill including a plurality of cutter elements. The cutter elements are typically formed of extremely hard materials and include a layer of polycrystalline diamond (PCD) or other superabrasive material such as cubic boron nitride, thermally stable diamond, polycrystalline cubic boron nitride, or ultrahard tungsten carbide. The mill is rotated and urged against the inside of the steel casing, thereby allowing the cutter elements to engage, penetrate, and shear small chips of the steel casing. This process is continued until the mill completely penetrates the steel casing.

The performance of conventional cutter elements cutting steel typically declines over time. In particular, thermal loads negatively impact cutter element life, and the development of wear flats on conventional cutter elements reduces cutting efficiency and effectiveness. Decreases in cutting performance typically results in an increase in milling time and associated costs.

BRIEF SUMMARY OF THE DISCLOSURE

These and other needs in the art are addressed in one embodiment by a drill bit for cutting through a downhole metal structure. In an embodiment, the bit comprises a bit body having a central axis and a bit face. The bit body is configured to rotate about the central axis in a cutting direction. In addition, the bit comprises a cutting structure disposed on the bit face. The cutting structure includes a plurality of circumferentially spaced blades and a plurality of primary cutter elements mounted to each blade. Each primary cutter element has a forward-facing primary cutting face. Each primary cutter element is made of a whisker ceramic composite.

These and other needs in the art are addressed in another embodiment by a cutting device for milling a downhole metal structure. In an embodiment, the cutting device comprises a body having a central axis, a first end coupled to a pin, and a second end defining an annular cutting face. In addition, the cutting device comprises a plurality of circumferentially-spaced cutter elements mounted to the cutting face. Each cutter element comprises a whisker ceramic composite.

These and other needs in the art are addressed in another embodiment by a method for sidetracking from a borehole. In an embodiment, the method comprises coupling a drill bit to a lower end of a drillstring. The drill bit comprises a bit body having a central axis and a bit face. The drill bit also comprises a cutting structure disposed on the bit face. The cutting structure includes a plurality of circumferentially spaced blades, a plurality of primary cutter elements mounted to each blade and a plurality of secondary cutter elements mounted to each blade. The secondary cutter elements on each blade trail the primary cutter elements on the same blade. Each cutter element has an extension height, and the extension height of each primary cutter element is greater than the extension height of each secondary cutter element. Each primary cutter element is made of a whisker ceramic composite. In addition, the method comprises (b) lowering the drill bit into a borehole lined with casing. Further, the method comprises (c) rotating the bit about the central axis in a cutting direction. Still further, the method comprises (d) engaging the casing with the cutting structure during (c). Moreover, the method comprises (e) milling the casing with the primary cutter elements during (d).

Embodiments described herein comprise a combination of features and advantages intended to address various shortcomings associated with certain prior devices, systems, and methods. The foregoing has outlined rather broadly the features and technical advantages of the invention in order that the detailed description of the invention that follows may be better understood. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description, and by referring to the accompanying drawings. It should be appreciated by those skilled in the art that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of the preferred embodiments of the invention, reference will now be made to the accompanying drawings in which:

FIG. 1 is a perspective view of an embodiment of drill bit in accordance with the principles described herein for milling a metal structure;

FIG. 2 is an enlarged cross-sectional view of one of the primary cutter elements of FIG. 1 illustrating the microstructure of the whisker ceramic composite forming the primary cutter elements of FIG. 1;

FIG. 3 is a graphical illustration of an embodiment of a method for performing a sidetracking operation with the drill bit of FIG. 1;

FIG. 4 is a side view of an embodiment of a cutting device in accordance with the principles described herein for milling a metal structure;

FIG. 5 is a partial cross-sectional view of the cutting device of FIG. 3;

FIG. 6 is an end view of the cutting face of FIG. 3;

FIGS. 7A-7D are schematic side views of embodiments of cutter elements in accordance with the principles described herein comprising whisker ceramic composites and having different, exemplary geometries; and

FIGS. 8A-8G are enlarged partial cross-sectional views of cutting devices illustrating exemplary techniques for attaching cutter elements comprising whisker ceramic composites thereto.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following discussion is directed to various exemplary embodiments. However, one skilled in the art will understand that the examples disclosed herein have broad application, and that the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to suggest that the scope of the disclosure, including the claims, is limited to that embodiment.

Certain terms are used throughout the following description and claims to refer to particular features or components. As one skilled in the art will appreciate, different persons may refer to the same feature or component by different names. This document does not intend to distinguish between components or features that differ in name but not function. The drawing figures are not necessarily to scale. Certain features and components herein may be shown exaggerated in scale or in somewhat schematic form and some details of conventional elements may not be shown in interest of clarity and conciseness.

In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . . ” Also, the term “couple” or “couples” is intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection, or through an indirect connection via other devices, components, and connections. In addition, as used herein, the terms “axial” and “axially” generally mean along or parallel to a central axis (e.g., central axis of a body or a port), while the terms “radial” and “radially” generally mean perpendicular to the central axis. For instance, an axial distance refers to a distance measured along or parallel to the central axis, and a radial distance means a distance measured perpendicular to the central axis.

Referring now to FIG. 1, an embodiment of a cutting device 10 for cutting or drilling through a downhole metal structure (e.g., steel casing, a packer, etc.) is shown. In this embodiment, device 10 is a fixed cutter bit, sometimes referred to as a drag bit. As will be described in more detail below, cutting device 10 can also be used to drill through an earthen formation such as immediately after cutting through casing during sidetracking operations. Accordingly, cutting device 10 may also be referred to as a mill or a bit. In this embodiment, cutting device 10 is a “torpedo” style fixed cutter bit, however, embodiments described herein are not limited to that specific type of fixed cutter bit.

Bit 10 includes a body 12, a shank 13 and a threaded connection or pin 14 for connecting bit 10 to a drill string (not shown), which is employed to rotate the bit in order to drill the metal structure. Body 12 includes a bit face 20, which supports a cutting structure 15 generally disposed on the end of the bit 10 that is opposite pin 14. Bit 10 has a central axis 11 about which bit 10 rotates in the cutting direction represented by arrow 18. Body 12 may be formed in a conventional manner using powdered metal tungsten carbide particles in a binder material to form a hard metal cast matrix. Alternatively, the body can be machined from a metal block, such as steel, rather than being formed from a matrix.

Body 12 may include bores and/or passages that permitting fluid (e.g., lubricating fluid, drilling fluid, etc.) to flow from the drill string into bit 10, and out of drill bit 10 through ports or nozzles disposed in bit face 20. Such bores and passages may serve to distribute fluid around cutting structure 15 to flush away metal cuttings during milling or formatting cuttings during drilling through the formation, and to remove heat from bit 10.

Referring still to FIG. 1, cutting structure 15 is provided on face 20 of bit 10 and includes a plurality of blades 16 extending along bit face 20. In this embodiment, the plurality of blades 16 are uniformly circumferentially-spaced about the bit face 20. Blades 16 are integrally formed as part of, and extend perpendicularly outwardly from body 12 and bit face 20. In addition, blades 16 extend generally radially across bit face 20 and longitudinally along a portion of the periphery of bit 10. Each blade 16 has a first or radially inner end 16 a at or proximal axis 11 and a second or radially outer end 16 b opposite end 16 a proximal shank 13. Blades 16 are separated by fluid flow courses 19.

Each blade 16 on bit face 20 provides a cutter-supporting surface 17 to which a plurality of cutter elements are mounted. In this embodiment, a plurality of primary cutter elements 40 having cutting faces 44 are mounted to cutter-supporting surface 17 of each blade 16, and a plurality of secondary cutter elements 50 having cutting faces 54 are mounted to cutter-supporting surface 17 of each blade 16. Primary cutter elements 40 are generally arranged in rows extending along each blade 16, and secondary cutter elements 50 are generally arranged in rows extending along each blade 16. However, secondary cutter elements 50 are positioned behind the primary cutter elements 40 provided on the same blade 16. Thus, when bit 10 rotates about central axis 11 in the cutting direction 18, secondary cutter elements 50 trail the primary cutter elements 40 provided on the same blade 16. Thus, as used herein, the term “secondary cutter element” is used to describe a cutter element that trails any other cutter element on the same blade 16 when bit 10 is rotated in the cutting direction represented by arrow 18. Further, as used herein, the term “primary cutter element” is used to describe a cutter element provided on the leading edge of a blade 16. In other words, when bit 10 is rotated about central axis 11 in the cutting direction 18 a “primary cutter element” does not trail any other cutter elements on the same blade 16. As will be described in more detail below, primary cutter elements 40 are sized, positioned, and configured to mill a window in steel casing, whereas secondary cutter elements 50 are sized, positioned, and configured to drill through the formation after milling through the casing.

In general, primary cutter elements 40 and secondary cutter elements 50 need not be positioned in rows, but may be mounted in other suitable arrangements provided each cutter element is either in a leading position (e.g., primary cutter element 40) or trailing position (e.g., secondary cutter element 50). Examples of suitable arrangements may include without limitation, rows, arrays or organized patterns, randomly, sinusoidal pattern, or combinations thereof.

In the embodiment shown in FIG. 1, primary cutter elements 40 and the secondary cutter elements 50 are mounted so that their cutting faces 44, 54, respectively, are forward facing. As used herein, “forward facing” is used to describe the orientation of a surface that is substantially perpendicular to or at an acute angle relative to the cutting direction 18 of bit 10. For instance, a forward facing cutting face 44, 54 may be oriented substantially perpendicular to the cutting direction of bit 10, may include a backrake angle, and/or may include a siderake angle.

Primary cutting faces 44 have a greater extension height than secondary cutting faces 54. As used herein, the term “extension height” is used to describe the distance a cutting face extends perpendicularly from the cutter-supporting surface of the blade to which it is attached. Thus, primary cutting faces 44 will contact the object being milled/drilled prior to secondary cutting faces 54, and generally provide a greater depth-of-cut than secondary cutting faces 54.

In this embodiment, each backup cutter element 50 is a conventional cutter element. In particular, each backup cutter element 50 comprises an elongated and generally cylindrical support member or substrate which is received and secured in a pocket formed in the surface of the blade 16 to which it is fixed, and each cutting face 54 comprises a forward facing disk or tablet-shaped, hard cutting layer of polycrystalline diamond or other superabrasive material is bonded to the exposed end of the corresponding support member.

In this embodiment, primary cutter elements 40 are generally cylindrical and mounted to blades 16, but are not conventional cutter elements and are not made of conventional cutter element materials. Rather, each primary cutter element 40 is made of a whisker ceramic composite 60 shown in more detail in FIG. 2.

Referring now to FIG. 2, an enlarged view of the microstructure of whisker ceramic composite 60 used to form cutter elements 40 is shown. Although whisker ceramic composite 60 is used to form cutter elements 40 of FIG. 1, in general, whisker ceramic composite 60 can be used to form other embodiments of cutter elements described herein as well as other types of cutter elements and cutting structures.

In general, a whisker ceramic composite comprises a ceramic matrix embedded with a plurality of distributed fibers or whisker reinforcements. As shown in FIG. 2, whisker ceramic composite 60 comprises a ceramic matrix 61 embedded with a plurality of distributed whiskers 62 that reinforce ceramic matrix 61. In embodiments described herein, ceramic matrix 61 preferably comprises aluminum-oxide or zirconium-oxide, and whiskers 62 preferably comprise silicon-carbide. In general, whiskers 62 can be uniformly distributed throughout ceramic matrix 61 or layered within the ceramic matrix 61. In addition, whether uniformly distributed or layered, whiskers 62 can be “oriented” parallel to the cutting plane, perpendicular to the cutting plane, or at an acute angle relative to the cutting plane. The orientation relative to the cutting plane can be varied between different layers as desired. As used herein, the term “cutting plane” refers to a plane oriented perpendicular to the cutting face (e.g., cutting face 44).

Experimental data and known material properties indicate that whisker ceramic composites including whisker ceramic composite 60 offer the potential for improved strength and toughness (e.g., resistance to fractures), improved resistance to thermal shock, and overall improved performance and durability cutting metals (e.g., steel) as compared to conventional cutter element materials such as polycrystalline diamond, cubic boron nitride, thermally stable diamond, polycrystalline cubic boron nitride, and tungsten carbide. For example, conventional cutter elements (e.g., secondary cutter elements 50) typically exhibit significant decreases in cutting effectiveness following the development of wear flats. However, cutter elements having cutting faces made of whisker ceramic composites (e.g., primary cutter elements 40) exhibit the ability to continue cutting effectively even after wear flats develop. In other words, whisker ceramic composites exhibit a “self-sharpening” characteristic that can continue to effectively cut metals even after significant wear. For instance, for testing purposes, an extremely large wear flat was intentionally formed on a cutter element cutting face made of a whisker ceramic composite, yet the cutter element continued to use the remaining material as a cutting edge. In addition, as previously described, conventional cutter elements (e.g., secondary cutter elements 50) include a cylindrical substrate and a forward facing tablet of hard cutting material bonded to the exposed end of the corresponding substrate. Thus, the effective volume of cutting material on a conventional cutter is limited to the tablet of hard cutting material. However, in embodiments described herein, the cutter elements are preferably entirely made of a whisker ceramic composite to increase and maximize the total volume of cutting material. For example, primary cutter elements 40 previously described and shown in FIG. 1 have a cylindrical body made of a homogenous whisker ceramic composite—the entire volume of each cutter element 40 is made of whisker ceramic composite 80.

As shown in FIG. 1, cutting faces 44 of whisker ceramic composite primary cutter elements 40 are planar. However, in other embodiments, the cutting faces of the whisker ceramic composite cutter elements (e.g., cutting faces 44) can have other geometries. Moreover, cutter elements made of whisker ceramic composites are particularly suited to a variety of different geometries as experimental data suggests that any exposed portion of a whisker ceramic composite cutter element can function as a cutting edge. In conventional cutter elements such as secondary cutter elements 50, the relatively thin table of ultrahard material forming the cutting face (e.g., cutting face 54) is bonded to the underlying substrate. Such a bonded connection between the table and the substrate can lead to delamination and chipping issues, particularly in cases where unconventional and exotic geometries are employed for the cutting face—residual stresses from processing the bonded layered composites limits it service loading. However, whisker ceramic composite cutter elements such as primary cutter elements 40, do not include layers bonded together, and thus, offer the potential for a greater variety of cutting face geometries.

Referring now to FIG. 3, an embodiment of a method 70 for sidetracking from a cased borehole with bit 10 of FIG. 1 is shown. In this embodiment, method 70 starts in block 71, where bit 10 is connected to the lower end of a drillstring and lowered downhole through the casing. Mill 10 is lowered into the cased borehole to the depth at which sidetracking is desired. Moving now to block 72, at the desired depth, mill 10 is employed to drill or mill through the casing to the surrounding formation. More specifically, mill 10 is rotated in cutting direction 18 while simultaneously being guided by a wedge or whipstock to engage the inside of the metal casing with cutting face 20. In this embodiment, whisker ceramic composite cutter elements 40 perform all or substantially all of the cutting of the metal casing, while the conventional secondary cutter elements 50 perform little, if any, cutting of the metal casing. In particular, whisker ceramic cutter elements 40 have extension heights greater than secondary cutter elements 50, and thus, engage the casing before secondary cutter elements 50. Consequently, the casing is cut primarily by the whisker ceramic composite cutter elements 40, whereas secondary cutter elements 50 do little cutting of the casing and are substantially preserved for drilling through the formation after milling through the casing.

Next, in block 73, bit 10 continues to cut through the metal casing, relying substantially or completely on whisker ceramic cutter elements 40, until a window is formed in the casing. In other words, bit 10 mills completely through the casing and into the surrounding formation. Bit 10 is designed to both mill through the casing, and then drill through the formation surrounding the casing without tripping. Thus, as shown in block 74, in this embodiment of method 70, bit 10 continues to be rotated in cutting direction 18 to engage the formation surrounding the casing with cutting face 20. As previously described, whisker ceramic composite cutter elements 40 have a greater extension height than secondary cutter elements 50. Thus, at least initially, whisker ceramic composite cutter elements 40 bear a significant cutting duty in the formation. Although whisker ceramic composite cutter elements 40 have improved toughness and are well-suited to cutting metals, they are generally less suited to cutting abrasive materials (e.g., the subterranean formation) as compared to secondary cutter elements 50. Thus, whisker ceramic composite cutter elements 40 quickly wear while drilling in the formation, thereby transferring the formation cutting duty to secondary cutter elements 50. Thus, secondary cutter elements 50 are preserved during milling of the metal casing in order to be used for drilling through the formation, and primary cutter elements 40 are used to mill the metal casing and sacrificed during drilling of the formation. In this manner, bit 10 leverages primary cutter elements 40 made of whisker ceramic composites, which provide enhanced performance in cutting metals, to mill the metal casing, and leverages secondary cutter elements 50 made of conventional cutter element materials to cut through the formation.

Although cutting device 10 is shown as a “torpedo” style fixed cuter bit, the use of whisker ceramic composite cutter elements 40 described herein is not limited to that particular type of fixed cutter bit. In general, embodiments of whisker ceramic composite cutter elements (e.g., primary cutter elements 40) can be used on any type of fixed cutter bit or mill known in the art. Although whiskers have been disclosed for use in a ceramic matrix, whiskers can also be used with other types of materials. For example, whiskers can be included in cubic boron nitride cutter elements more adept at cutting rock and earthen formations.

Referring now to FIGS. 4-6, an embodiment of a cutting device 100 for cutting or drilling through a downhole metal structure (e.g., steel casing, a packer, etc.) is a mill shoe. In this embodiment, cutting device 100 is designed to mill a downhole metal object or structure such as casing or a packer. Cutting device 100 has a central or longitudinal axis 105 and includes a body 110 and a threaded connection or pin 14 for connecting cutting device 100 to a drill string (not shown), which is employed to rotate device 100 about axis 105 in a cutting direction 108. Body 110 has a first or upper end 110 a attached to pin 14, a second or lower end 110 b opposite end 110 a, and a through bore or passage 111 extending axially between ends 110 a, b. Lower end 110 b defines an annular cutting face 112, which supports a cutting structure 113 designed to engage and cut a downhole metal structure or object. In this embodiment, body 112 is general cylindrical, however, in other embodiments, the body (e.g., body 112) may have other shapes. In general, body 110 can be formed using powdered metal tungsten carbide particles infiltrated with a binder material to form a hard metal composite cast matrix or is machined from a metal block, such as steel.

During milling operations, fluid (e.g., lubricating fluid, drilling fluid, etc.) is pumped down the drillstring, through pin 14 and bore 111, and out of body 110 at end 110 b. Such fluid is distributed around cutting structure 113 and serves to flush away metal cuttings during milling and to remove heat from cutting device 100.

Referring still to FIGS. 4-6, cutting structure 113 is provided on cutting face 112 and includes a plurality of circumferentially adjacent cutter elements 120 mounted to lower end 110 b. Cutting elements 120 extend axially from face 112 and lower end 110 b, and are designed to engage, cut, shear, and chip the metal being milled. In this embodiment, cutter elements 120 are rectangular prisms, however, as will be described in more detail below, other geometries can also be employed such as cylindrical, triangular, etc. In addition, cutter elements 120 are not made of conventional cutter element materials such as polycrystalline diamond or tungsten carbide. Rather, each cutter element 120 is made of a whisker ceramic composite. More specifically, in this embodiment, each cutter element 120 is made of whisker ceramic composite 60 previously described. A variety of exemplary methods for securing whisker ceramic cutter elements 120 to metal or metal matrix body 110 will be described in more detail below. As previously described, the ceramic matrix 61 of whisker ceramic composite 60 preferably comprises aluminum-oxide or zirconium-oxide, and the whiskers 62 of whisker ceramic composite 60 preferably comprise silicon-carbide. In general, the whiskers 62 in each cutter element 120 can be uniformly distributed throughout the ceramic matrix 61 or layered within the ceramic matrix 61. In addition, whether uniformly distributed or layered, the whiskers 62 in each cutter element 120 can be “oriented” parallel to the cutting plane, perpendicular to the cutting plane, or at an acute angle relative to the cutting plane. The orientation relative to the cutting plane can be varied between different layers as desired. As previously described, experimental data and known material properties indicate whisker ceramic composites such as composite 60 offer the potential for improved strength and toughness (e.g., resistance to fractures), improved resistance to thermal shock, and overall improved performance and durability cutting metals (e.g., steel) as compared to conventional cutter element materials such as polycrystalline diamond, cubic boron nitride, thermally stable diamond, polycrystalline cubic boron nitride, and tungsten carbide.

Referring now to FIGS. 7A-7D, exemplary whisker ceramic composite cutter elements 130, 140, 150, 160 that can be used in place of cutter elements 120 on cutting device 100 previously described, or with other types of cutting devices, drill bits, and mills are shown. Each cutter element 130, 140, 150, 160 is shown moving in a cutting direction 121 to engage and cutting an exemplary metal structure 122. In these embodiments, each cutter element 130, 140, 150, 160 is made from whisker ceramic composite 60 previously described, however, it should be appreciated that other whisker ceramic composites can be used to form cutter elements 130, 140, 150, 160.

Referring first to FIG. 7A, whisker ceramic composite cutter element 130 has a central axis 135, and includes a base portion 131 and a cutting portion 132 extending therefrom. Cutting portion 132 includes a cutting surface 133. Collectively, base 131 and cutting portion 132 define the overall height of cutter element 130. In this embodiment, cutter element 130 is generally a rectangular prism although other geometries can be employed. Base portion 131 is secured to the cutting device (e.g., cutting device 100) such that cutting portion 132 and cutting surface 133 extend beyond the body of the cutting device for engaging the metal structure 122. In this embodiment, cutting portion 132 includes a plurality of elongate parallel spaced grooves or recesses 134 that define a plurality of elongate parallel cutting teeth 136 therebetween along cutting surface 133. Grooves 134 and teeth 136 are oriented perpendicular to cutting direction 121. Each tooth 136 has a height H₁₃₆ measured axially from base portion 131 to the outermost tip of the tooth 136. In this embodiment, H₁₃₆ is preferably between 0.020 in. and 0.040 in.

Spaced teeth 136 define a plurality of laterally spaced apart parallel cutting edges 137 along cutting surface 133. Such edges 137 are arranged one-behind-the-other relative to the cutting direction 121. During cutting operations, the leading cutting edge 137 (relative to the cutting direction 121) shears metal structure 121, and the trailing cutting edges 137 (relative to the cutting direction 121) help break up the shaved cuttings and chips from metal structure 121. Further, in the event the leading cutting edge 137 gets damaged, breaks, or chips, the next cutting edge 137 (relative to the cutting direction 121) can take over the primary cutting duties. In this sense, cutter element 130 is self-sharpening. The internal corners within grooves 134 as well as the external edges of teeth 136 (e.g., cutting edges 137) can be radiused (preferably at least a 0.1 mm radius) as desired to reduce stress concentrations and enhance durability in service.

Referring now to FIG. 7B, whisker ceramic composite cutter element 140 is substantially the same as cutter element 130 previously described. Namely, cutter element 140 has a central axis 145, and includes cutting portion 132 as previously described and a base portion 141 from which cutting portion 132 extends. However, in this embodiment, base portion 141 includes a flange 142 opposite cutting portion 132. Flange 142 extends radially outward and extends along the entire periphery of base portion 141. As will be described in more detail below, flange 142 functions as a retention mechanism for securing cutter element 140 to the associated cutting device (e.g., cutting device 100). The intersection between flange 142 and the remainder of base portion 141 and the radially outermost edge 143 of flange 142 can be radiused (preferably at least a 0.1 mm radius) as desired to reduce stress concentrations and enhance durability in service.

Referring now to FIG. 7C, whisker ceramic composite cutter element 150 has a central axis 155, and includes a base portion 151 and a cutting portion 152 extending therefrom. Collectively, base 151 and cutting portion 152 define the overall height of cutter element 150. Cutting portion 152 includes a cutting surface 153. In this embodiment, cutter element 150 is cylindrical, and thus, has an outer diameter D₁₅₀ that is preferably between 10.0 and 15.0 mm. In this embodiment, diameter D₁₅₀ is 13.0 mm. Although cutter element 150 is cylindrical in this embodiment, in general, the cutter element (e.g., cutter element 150) may be formed in a variety of shapes other than cylindrical. In this embodiment, cutter element 150 is disposed at a backrake angle α. In general, the backrake angle of a cutter element is the angle formed between the central axis of the cutter element and the normal vector of the surface of the material being cut (i.e., the vector oriented perpendicular to the surface of the material being cut). Thus, backrake angle α of cutter element 150 is the angle measured between axis 155 of cutter element 150 and the normal vector V of the surface of material 122 being cut. In this embodiment, backrake angle α is preferably between 5° and 20°. In more demanding applications, the backrake angle α can be greater than 20°.

Base portion 151 is secured to the cutting device (e.g., cutting device 100) such that cutting portion 152 and cutting surface 153 extend beyond the body of the cutting device for engaging the metal structure 122. In this embodiment, cutting portion 152 includes a plurality of steps 154 that define a plurality of cutting edges 156 a, b. The inner corners between steps 154 and cutting edges 156 a, b can be radiused (preferably at least a 0.1 mm radius) as desired to reduce stress concentrations and enhance durability in service. Leading step 154 and corresponding cutting edge 156 a extends to a height H_(156a) measured axially from base portion 151, and trailing step 154 and corresponding cutting edge 156 b extends to a height H_(156b) measured axially from leading step 154. In general, height H_(156a) and height H_(156b) can be the same or different. In this embodiment, height H_(156a) and height H_(156b) are each 0.5 mm. Further, in this embodiment, each step 154 has a length L₁₅₄ measured perpendicular to axis 155 equal to 1.0 mm.

Cutting portion 152 and cutting surface 153 define a total depth-of-cut (DOC) D_(t), however, the total DOC D_(t) is divided and shared between cutting edges 156 a, b. In other words, leading cutting edge 156 a engages metal structure 121 to a first DOC D₁, and trailing cutting edge 156 b engages metal structure 121 to a DOC D₂, and thus, neither cutting edge 154 experiences the total DOC D_(t).

Referring now to FIG. 7D, whisker ceramic composite cutter element 160 has a central axis 165, and includes a base portion 161 and a cutting portion 162 extending therefrom. Cutting portion 162 includes a cutting surface 163. In this embodiment, cutter element 160 is generally a rectangular prism although other geometries can be employed. Collectively, base 161 and cutting portion 162 define the overall height of cutter element 160. Base portion 161 is secured to the cutting device (e.g., cutting device 100) such that cutting portion 162 and cutting surface 163 extend beyond the body of the cutting device for engaging the metal structure 122.

In this embodiment, axis 165 is parallel to the normal vector V, and thus, cutter element 160 is not disposed at a backrake angle. However, cutting surface 163 is generally sloped relative to the surface of material 122 being cut, thereby resulting in an effective backrake. In particular, moving in the opposite direction of cutting direction 121 from a leading side 160 a of cutter element 160 to a trailing side 160 b of cutter element 160, the height of cutting surface 163 measured axially from base portion 161 generally increases, thereby creating the effective backrake. However, between sides 160 a, 160 b, cutting surface 163 includes a random arrangement of recesses 166 and peaks 167 defining a plurality of cutting edges for engaging and cutting metal structure 122. As any peak 167 becomes damaged or break, another peak 167 and associated cutting edge can take on cutting duties. The random arrangement of cutting edges of cutter element 160, and associated random cutting effect, may be particularly suited for use in connection with impregnated bits. As is known in the art, an impregnated bit, or simply an “impreg” bit, is a bit having a cutting face impregnated with a plurality of diamonds that engage and cut a material by a grinding action as opposed to a shearing action. As an alternative to or in addition to diamonds mounted to the cutting face of an impreg bit, a plurality of cutter elements 160 can be secured to the impreg bit with cutting surfaces 163 extending from the bit face for engaging and grinding the material being cut.

As previously described, whisker ceramic composite cutter elements 40 are securely attached to blades 16 and whisker ceramic composite cutter elements 120 are securely mounted to lower end 110 b of cutting device 100. In general, embodiments of cutter elements comprising whisker ceramic composites (e.g., cutter elements 40, 120, 130, 140, 150, 160) can be secured to the body of the underlying cutting device by any suitable means known in the art. For example, embodiments of whisker ceramic composite cutter elements described herein can be securely attached to an underlying metal using known techniques for brazing ceramics to metals such as microwave brazing techniques and metallising and active braze techniques. Additional techniques for securely attaching cutter elements comprising whisker ceramic composites to an underlying cutting device are schematically illustrated in FIGS. 8A-8E. In general, any of the attachment means disclosed in FIGS. 8A-8F and described in more detail below can be employed to secure any cutter element described herein, such as any of cutter elements 40, 120, 130, 140, 150, 160 previously described, to a cutting device (e.g., drill bit, mill, etc.).

Referring first to FIG. 8A, a cutting device 200 (e.g., a mill or a drill bit) includes a body 201 and a cutter element 210 rigidly secured to the underlying body 201. In this embodiment, cutter element 210 includes a metal (e.g., steel) substrate or base 211 and a whisker ceramic composite cutting layer 212 attached to base 211. In addition, in this embodiment, cutting layer 212 is made of whisker ceramic composite 60 and is rigidly and securely mounted to base 211 via known brazing techniques. Base 211 includes a throughbore 213 extending therethrough, and cutting layer 212 includes a throughbore 214 extending therethrough and coaxially aligned with bore 213. A bolt 215 is advanced through bores 213, 214 and is threaded into a mating internally threaded receptacle in body 201, thereby removably securing cutter element 210 to body 201 of cutting device 200. Use of bolt 215 to attached cutter element 210 to body 201 enables cutter elements 210 to be serviced and maintained with relative ease as worn or damaged cutter elements 210 can be removed and replaced by unbolting them from body 201 and then bolting new cutter elements 210 to body 201. This may also provide a means to retrofit existing cutting devices with cutter elements comprising whisker ceramic composite materials.

Referring now to FIG. 8B, a cutting device 300 (e.g., a mill or a drill bit) includes a body 301 and a whisker ceramic composite cutter element 310 rigidly secured to the underlying body 301 with a metal sleeve or jacket 311. In this embodiment, cutter element 310 is made entirely of whisker ceramic composite 60 and is secured within jacket 311 via interference fit. A combination of press and/or shrink fit can be used to obtain desired interference. For example, the metal sleeve 311 can be heated, the cutter element 310 slidably disposed therein, and then the metal sleeve 311 allowed to cool and shrink into engagement with cutter element 310. In this embodiment, an interference of about 0.5 mm. between cutter element 310 and sleeve 311 is provided. The exposed end of cutter element 310 defines a cutting surface or face 312. Metal jacket 311 is preferably made of hardened steel (e.g., 4140 H.T with a yield strength greater than 110 ksi), tool steel (e.g., A2 or H2), a superalloy (e.g., Inconel), or heavy metal (e.g., Mo and W).

It should be appreciated that the interference fit desirably places the whisker ceramic composite 60 forming cutter element 310 in compression, which offers the potential to enhance impact resistance. With cutter element 310 securely disposed within sleeve 311, sleeve 311 is brazed to body 301 of cutting device 300 using conventional brazing techniques. In this embodiment, sleeve 311 has an open end 311 a that receives cutter element 310 and a closed end 311 b against which cutter element 310 is seated. Closed end 311 b includes a relief port or hole 313. However, in other embodiments, the metal sleeve is opened at both ends.

Referring now to FIG. 8C, a cutting device 400 (e.g., a mill or a drill bit) includes a body 401 and a whisker ceramic composite cutter element 410 rigidly secured to the underlying body 401 with a metal sleeve or jacket 411. In this embodiment, cutter element 410 is made entirely of whisker ceramic composite 60 and is secured within jacket 411 via interference fit. Sleeve 411 is the same as sleeve 311 previously described except that, in this embodiment, both ends 411 a, 411 b of sleeve 411 are open, and further, sleeve 411 covers the entire periphery of cutter element 410. In other words, cutter element 410 does not extend from sleeve 411. Cutter element 410 has a cutting surface or face 412 at one end 411 a of sleeve 411 and a back face 413 at the opposite end 411 b of sleeve 411. With cutter element 410 securely disposed within sleeve 411, sleeve 411 is brazed to body 401 of cutting device 400 using conventional brazing techniques.

Referring now to FIG. 8D, a cutting device 500 (e.g., a mill or a drill bit) includes a body 501 and a whisker ceramic composite cutter element 510 rigidly secured to the underlying body 501 with a metal sleeve or jacket 511. In this embodiment, cutter element 510 is made entirely of whisker ceramic composite 60 and is secured within sleeve 511 via interference fit. Sleeve 511 is the same as sleeve 411 previously described except that, in this embodiment, the inner surface of sleeve 511 engaging cutter element 510 tapers inward moving from end 511 b to end 511 a. The tapered or negative draft of sleeve 511 offers the potential for enhanced retention and mechanical lock of cutter element 510 therein. Cutter element 510 has a cutting surface or face 512 at one end 511 a of sleeve 511 and a back face 513 at the opposite end 511 b of sleeve 511. With cutter element 510 securely disposed within sleeve 511, sleeve 511 is brazed to body 501 of cutting device 500 using conventional brazing techniques.

Referring now to FIG. 8E, a cutting device 600 (e.g., a mill or a drill bit) includes a body 601 and a whisker ceramic composite cutter element 610 rigidly secured to the underlying body 601. In this embodiment, cutter element 610 is made entirely of whisker ceramic composite 60 and is brazed to a metal (e.g., steel) base 611 using a filler material 612 and conventional brazing techniques. The filler material 612 provides a transition layer between the whisker ceramic composite cutter element 610 and metal base 611 to enhance the connection therebetween. In addition, in this embodiment, filler material 612 provides additional cutting contact points when cutter element 610 is completely worn down to filler material 612. In particular, crushed carbide is distributed throughout a binder in filler material 612. The crushed carbide provides an added cutting mechanism once exposed. With cutter element 610 securely attached to base 611 with filler material 612, base 611 is brazed to body 601 of cutting device 600 using conventional brazing techniques.

Referring now to FIG. 8F, a cutting device 700 (e.g., a mill or a drill bit) includes a body 701 and a whisker ceramic composite cutter element 710 rigidly secured to the underlying body 701 with a braze material 711. In this embodiment, cutter element 710 is made of whisker ceramic composite 60 pre-coated with braze material 711, which includes an “active” brazing component for subsequent brazing to body 701 by means known in the art such as vacuum furnace methods, argon methods, or JPL microwave brazing methods. Examples of active braze materials that can be used for braze material 711 include titanium, TicuSil® brazing alloy available from Morgan Technical Ceramics of Hayward, Calif., and IncuSil®-ABA™ brazing alloy available from available from Morgan Technical Ceramics of Hayward, Calif.

Referring now to FIG. 8G, a cutting device 800 (e.g., a mill or a drill bit) includes a body 801 and a whisker ceramic composite cutter element 810 rigidly secured to the underlying body 801. In this embodiment, cutter element 810 is made entirely of whisker ceramic composite 60 and includes a cutting portion 811 at a first end 810 a, a base portion 812 extending from an end 810 b to cutting portion 811, and a retention flange 813 extending around the periphery of base portion 812 at end 810 b. Cutter element 810 is precast into the matrix 814 of the body 801. The matrix 814 surrounds retention flange 813, thereby securing cutter element 810 to body 801. As previously described with respect to cutter element 140 of FIG. 7B, the outer edge 815 of flange 813 can be radiused to reduce stress concentrations.

In the manner described, cutter elements comprising whisker ceramic composites can be securely attached to cutting devices for milling a downhole metal object or structure such as casing or a packer. Experimental data and known material properties indicate such whisker ceramic composites offer the potential for improved strength and toughness (e.g., resistance to fractures), improved resistance to thermal shock, and overall improved performance and durability cutting metals (e.g., steel) as compared to conventional cutter element materials such as polycrystalline diamond, cubic boron nitride, thermally stable diamond, polycrystalline cubic boron nitride, and tungsten carbide. Accordingly, embodiments of cutter elements described herein offer the potential for improved metal cutting performance, speed, and durability as compared to conventional cutter elements.

While preferred embodiments have been shown and described, modifications thereof can be made by one skilled in the art without departing from the scope or teachings herein. The embodiments described herein are exemplary only and are not limiting. Many variations and modifications of the systems, apparatus, and processes described herein are possible and are within the scope of the invention. For example, the relative dimensions of various parts, the materials from which the various parts are made, and other parameters can be varied. Accordingly, the scope of protection is not limited to the embodiments described herein, but is only limited by the claims that follow, the scope of which shall include all equivalents of the subject matter of the claims. Unless expressly stated otherwise, the steps in a method claim may be performed in any order. The recitation of identifiers such as (a), (b), (c) or (1), (2), (3) before steps in a method claim are not intended to and do not specify a particular order to the steps, but rather are used to simplify subsequent reference to such steps. 

1. A drill bit for cutting through a downhole metal structure, the bit comprising: a bit body having a central axis and a bit face, wherein the bit body is configured to rotate about the central axis in a cutting direction; a cutting structure disposed on the bit face, wherein the cutting structure includes a plurality of circumferentially spaced blades, a plurality of primary cutter elements mounted to each blade, and a plurality of secondary cutter elements mounted to each blade, wherein the primary cutter elements on each blade lead the secondary cutter elements on the same blade relative to the cutting direction, wherein each primary cutter element has a forward-facing primary cutting face and each secondary cutter element has a forward-facing secondary cutting face; wherein each primary cutter element has an extension height that is greater than an extension height of each secondary cutter element; wherein each secondary cutter element comprises a substrate and an ultrahard table mounted to the substrate; wherein each primary cutter element is made of a whisker ceramic composite; wherein the whisker ceramic composite is configured to cut the downhole metal structure and the ultrahard table is configured to cut a subterranean formation adjacent the downhole metal structure.
 2. (canceled)
 3. The drill bit of claim 1, wherein the primary cutter elements on each blade are arranged in a row extending along the blade; and wherein the secondary cutter elements on each blade are arranged in a row extending along the blade.
 4. (canceled)
 5. (canceled)
 6. The drill bit of claim 1, wherein each primary cutter element is made of a ceramic matrix and a plurality of silicon-carbide whiskers distributed throughout the ceramic matrix.
 7. The drill bit of claim 6, wherein the ceramic matrix is an aluminum-oxide or zirconium oxide.
 8. The drill bit of claim 1, wherein each primary cutter element is brazed to the corresponding blade or pre-cast with the bit body.
 9. The drill bit of claim 1, wherein each primary cutter element is secured within a sleeve by an interference fit, and wherein each sleeve is brazed to the corresponding blade.
 10. A cutting device for milling a downhole metal structure, the cutting device comprising: a body having a central axis, a first end coupled to a pin, and a second end defining an annular cutting face; a plurality of circumferentially-spaced cutter elements mounted to the cutting face, wherein each cutter element comprises a whisker ceramic composite.
 11. The cutting device of claim 10, wherein each cutter elements has a cylindrical or rectangular geometry.
 12. The cutting device of claim 10, wherein the body includes a throughbore extending axially therethrough from the first end to the second end.
 13. The cutting device of claim 10, wherein each cutter element is made of a ceramic matrix and a plurality of silicon-carbide whiskers distributed throughout the ceramic matrix.
 14. The cutting device of claim 13, wherein the ceramic matrix is an aluminum-oxide or zirconium oxide.
 15. The cutting device of claim 10, wherein each cutter element has a base portion secured to the body and a cutting portion with a cutting surface, wherein the cutting surface of each cutter element includes a plurality of steps.
 16. The cutting device of claim 15, wherein each cutter element is disposed at a backrake angle between 5° and 20°.
 17. The cutting device of claim 10, wherein each cutter element has a base portion secured to the body and a cutting portion with a cutting surface, wherein the cutting surface of each cutter element includes a plurality of parallel teeth.
 18. The cutting device of claim 10, wherein each cutter element is brazed to the body or removably mounted to the body with a bolt.
 19. A method for sidetracking from a borehole, the method comprising: (a) coupling a drill bit to a lower end of a drillstring, wherein the drill bit comprises: a bit body having a central axis and a bit face; a cutting structure disposed on the bit face, wherein the cutting structure includes a plurality of circumferentially spaced blades, a plurality of primary cutter elements mounted to each blade and a plurality of secondary cutter elements mounted to each blade, wherein the secondary cutter elements on each blade trail the primary cutter elements on the same blade; wherein each cutter element has an extension height and a forward-facing cutting face, and wherein the extension height of each primary cutter element is greater than the extension height of each secondary cutter element; wherein the forward-facing cutting face of each primary cutter element is made of a whisker ceramic composite configured to engage and cut casing lining a borehole; wherein the forward-facing cutting face of each secondary cutter element is made of an ultrahard table mounted to a substrate, wherein the ultrahard table is configured to engage and cut a borehole in an earthen formation; (b) lowering the drill bit into a borehole lined with casing; (c) rotating the bit about the central axis in a cutting direction; (d) engaging the casing with the cutting structure during (c); (e) milling the casing with the primary cutter elements during (d); (f) cutting a hole through the casing with the drill bit; (g) advancing the drill bit through the hole in the casing; and (h) drilling a borehole in an earthen formation with the secondary cutter elements.
 20. (canceled)
 21. The method of claim 19, further comprising sacrificing the primary cutter elements during (h).
 22. The drill bit of claim 1, wherein the whisker ceramic composite of each primary cutter element comprises a ceramic matrix embedded with a plurality of distributed fibers, wherein at least some of the plurality of distributed fibers are oriented perpendicular to the primary cutting face. 